Cindy Batchelor, Executive General Manager, NAB Business said a “power issue … isolated our mainframe”, an “incredibly rare event” that “took a number of hours for our technicians to be able to bring our systems back up”.

When Batchelor spoke to journalists on Sunday, the bank didn't yet have an assessment of how many customers were affected by the outage.

The sketchiness of the information given to Batchelor didn't help her deliver any clarity about the root cause of the outage, but she made it clear that more than just the mainframe was involved.

“It was a series of failures – as I said – that was driven by a power issue and it took our technicians a number of hours to actually get underneath what the issue was and then start to restore services one-by-one”, she said.

“So when the power goes into the systems, there was a disconnect there that impacted a number of our systems, which provide services to our customers.”

Of the Twitter speculation that inevitably followed the outage, perhaps the most cogent suggestion for the chain of events came from Conor Stokes, who suggested: “I imagine this scenario: 1) the main online transaction processing server fails somehow 2) fail over to the hot replica doesn't work 3) your phone system goes down because it queries your online transaction processing server for account queries.”

Batchelor reassured customers that the outage didn't result from any kind of security breach: “No personal data has been compromised”, she said.

Batchelor also said the bank will work through losses with customers, and will be offering compensation: “Our intention is to work with each and every one of them to make sure they have no financial loss associated with the outage today.” ®